Redefining the Second

In late November, Alan Madej, a clockmaker of sorts, was testing a new timepiece in his workshop. At the heart of the clock, which occupies two rooms, is a single ion of strontium—a pliable metal known for its ruby incandescence in fireworks, and number thirty-eight on the periodic table. The ion is trapped by lasers and held perfectly centered in a small vacuum, where it is shielded from magnetic fields and cooled to a fraction of a degree above absolute zero. “The ion itself must remain as isolated as possible from the external universe,” Madej, a senior research officer at Canada’s National Research Council, explained. The result, a new kind of atomic clock, is one of the most accurate timekeeping devices ever created.

Most of today’s atomic clocks keep time by counting high-frequency microwaves. To set that frequency, metrologists, as scientists of measurement are known, exploit a law of quantum physics: atoms become excited, and their structure changes, when they are exposed to specific amounts of energy—and the amount of energy required to cause a transition from one particular state to another is immutable, specific to each element. It’s “nature’s ruler,” Madej said. Cesium, the element most commonly used in high-precision atomic clocks, is nudged into an excited state at a microwave frequency of just under 9.2 billion cycles per second. In a cesium-based atomic clock, each cycle can be thought of as a pendulum swing.

A number of metrology labs around the world, like Madej’s, have been developing a new generation of atomic timepieces, known as optical, or optical-lattice, clocks. Because these clocks rely not on microwaves but on lasers, which operate at far higher frequencies, they can split each second into more intervals. Madej’s single-ion clock measures the four hundred and thirty trillion light waves per second required to energize a strontium atom. It converts this to four hundred and thirty trillion “ticks,” parsing time more finely than any of today’s microwave clocks.

The competition between these labs has evolved into something “like an Olympics of time standards,” Madej said. A recent test by the National Institute of Standards and Technology demonstrated an optical clock, running on the rare-earth metal ytterbium, that breaks every second into more than five hundred trillion intervals.

“Exquisitely precise timing is built into every aspect of modern infrastructure,” Thomas O’Brian, chief of the Time and Frequency Division at N.I.S.T., told me. “G.P.S., smartphones, computer networks, the Internet, electrical power—these all require synchronization down to the billionth of a second.” The steady backbeat of atomic clocks is what makes them work. Take global positioning systems. When turned on, a G.P.S. receiver triangulates its location using signals from overhead satellites. These satellites, outfitted with small atomic clocks, stamp every signal transmission down to the nanosecond, or billionth of a second. Because the speed of the satellite’s signal is known—approximately a foot per nanosecond—by measuring the precise amount of time it spends in transit to the receiver, a ground location can be derived using the satellites as reference points. If we could only measure seconds by the millionths—the length of time it takes a signal to travel a thousand feet—G.P.S. estimates could be off by miles. It would be akin to designing a microchip with a measuring tape.

If, by measuring time down to the billionth of a second, the G.P.S. in our phones can find us within a few metres, do we really need to scale down further, to the trillionth? “The actual technology applications are never obvious at the outset,” Madej said. “When atomic time was first introduced through cesium clocks, do you think that we imagined it would guide our cars?”

Measuring time with such finesse is a relatively recent development in human history. Minutes and seconds could not be accurately assessed until the seventeenth century, when the pendulum clock was invented. The Italian polymath Tito Livio Burattini proposed one of the first formal definitions of the second, 1/86,400 of a solar day, in his 1685 treatise on metrology. But the rotation of the earth is an unreliable touchstone: churning magma in the planet’s core, the growth and shrinkage of sea ice, and wind battering mountains all lead to microseconds of variation in the duration of its daily rotation. The gravitational pull of the moon and related tidal shifts also impose a steady brake on the earth’s spin, making the days of 2013 roughly two milliseconds longer than the days of 1900. Because earth’s rotation is irregular, a second based upon its rotation is also irregular.

Such imprecision became problematic when quantum mechanics and Einstein’s theory of relativity ruptured twentieth-century physics. Experimental verification of these theories demanded a steadier unit of time, as did emerging technologies like radio broadcasting and electricity distribution. Lord Kelvin was the first to theorize about timekeepers based on atomic energy levels. In January, 1945, the Columbia University professor and Nobel Laureate Isidor Rabi laid out the engineering basics of this theory in a public lecture. The next morning’s Times exulted in the prospect: “Blueprints for the most accurate clock in the universe, tuning in on radio frequencies in the hearts of atoms and thus beating in harmony with the ‘cosmic pendulum,’ were outlined yesterday.” Within a few years, the U.S. National Bureau of Standards had constructed a prototype atomic clock, and by 1955, England’s National Physical Laboratory unveiled the first cesium-based model, which ultimately changed the way we measure time.

In October of 1967, after more than a decade of negotiation, representatives from thirty-six countries convened in Paris for the thirteenth General Conference on Weights and Measures. Within the week, they had hammered out, in a single sentence, a new basis for the international second: “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.” The base unit of time became decoupled from the uneven pirouette of earth, defined instead by an intrinsic atomic property.

Today’s optical clocks will necessitate a new, third definition of the second, based on laser frequencies and, potentially, an element other than cesium. The question of which particular element will define the next second—strontium? ytterbium?—remains open. “We’ll compare each of these new clocks against each other, and likely a clear candidate will emerge down the road,” Madej said. He meant the 2019 General Conference, or even the one after that, in 2023. “The world metrology community doesn’t want to jump into things,” he added. “We’re a bit of a cautious group.”

Photograph by Andrew Brookes/Science Photo Library/Corbis.

Correction: The description of the ytterbium clock was updated to be more accurate.

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